Methicillin Resistant Staphylococcus Aureus Pharmacotherapy

Methicillin-resistant Staphylococcus aureus (MRSA) carries mecA (or mecC), which encodes PBP2a with low affinity for almost all beta-lactams. Healthcare-associated strains are multiresistant; community-associated strains are often susceptible to non-beta-lactams and may carry the Panton-Valentine leucocidin gene, causing recurrent skin abscesses and necrotising pneumonia. Vancomycin-intermediate (VISA), heteroresistant (hVISA) and vancomycin-resistant (VRSA) strains are uncommon but important.

Treatment principles. Source control (drainage of abscess, removal of infected devices, debridement) is as important as antibiotic choice. Use bactericidal drugs for bacteraemia and endocarditis. Obtain repeat blood cultures every 24 to 48 hours until negative. Do not use rifampicin or fusidic acid alone.

• Glycopeptides: vancomycin (first line for serious infection), teicoplanin.
• Lipopeptide: daptomycin (not for pneumonia).
• Oxazolidinones: linezolid and tedizolid (pneumonia, skin, bone; oral step-down).
• Cephalosporins: ceftaroline, ceftobiprole.
• Lipoglycopeptides: dalbavancin, oritavancin, telavancin.
• Oral options for mild or step-down therapy: co-trimoxazole, doxycycline or minocycline, clindamycin (if the D-test is negative), rifampicin (only in combination).
• Adjuncts: fosfomycin and gentamicin in selected combinations; clindamycin or linezolid for toxin suppression.

Adult regimens by syndrome (normal renal function).

• Skin abscess: incision and drainage; antibiotics for fever, extensive cellulitis, immunosuppression or incomplete drainage. Oral co-trimoxazole 960 mg every 12 hours, doxycycline 100 mg every 12 hours, or clindamycin 300 to 450 mg every 6 to 8 hours, for 5 to 10 days; linezolid 600 mg every 12 hours for severe disease.
• Bacteraemia: vancomycin (load 20 to 35 mg/kg, then adjust to an AUC of 400 to 600 mg.h/L) or daptomycin 8 to 10 mg/kg once daily. Remove the focus and any catheter. Duration at least 14 days from the first negative culture for uncomplicated disease (no endocarditis, no prosthetic material, negative follow-up cultures within 2 to 4 days, fever settled by 72 hours, no metastatic focus), and 4 to 6 weeks for complicated disease.
• Native valve endocarditis: vancomycin or daptomycin 8 to 10 mg/kg for 6 weeks. Prosthetic valve: vancomycin with rifampicin and gentamicin, specialist-led.
• Pneumonia: linezolid 600 mg every 12 hours or vancomycin; 7 days when responding, longer in empyema or necrosis; add clindamycin or linezolid for necrotising pneumonia with toxin production.
• Bone and joint infection: vancomycin or daptomycin for the early intravenous phase, then oral step-down (linezolid, co-trimoxazole with rifampicin, doxycycline with rifampicin) for a total of 6 weeks in osteomyelitis; prosthetic joint infection with retained hardware: rifampicin added after bacteraemia clears.
• CNS infection: vancomycin (target trough or AUC at the upper end) or linezolid, plus source removal.
• Persistent bacteraemia (over 72 hours): confirm source control, check vancomycin exposure, switch to daptomycin 10 mg/kg, and add ceftaroline 600 mg every 8 hours or fosfomycin, under specialist advice. Avoid vancomycin when the MIC is 2 mg/L or more.
• Children: vancomycin 15 mg/kg every 6 hours; clindamycin 30 to 40 mg/kg daily in divided doses for non-severe disease; linezolid 10 mg/kg every 8 hours.

Selection. Choose by site, severity and local susceptibility. Reserve daptomycin and linezolid for failure, renal risk, intolerance or oral step-down.

Principal risks. Vancomycin nephrotoxicity (higher with piperacillin-tazobactam) and infusion reaction; daptomycin myopathy; linezolid myelosuppression, neuropathy and serotonin syndrome; co-trimoxazole hyperkalaemia and marrow suppression; clindamycin C. difficile infection; rifampicin interactions.

Monitoring. Vancomycin levels or AUC after the fourth dose; creatinine every 2 to 3 days; creatine kinase weekly with daptomycin; full blood count weekly with linezolid or co-trimoxazole; echocardiography in S. aureus bacteraemia (transthoracic in all, transoesophageal in high-risk or prolonged bacteraemia); clinical review at 72 hours.

Special groups. Pregnancy: vancomycin and clindamycin are acceptable; avoid doxycycline, linezolid (unless essential) and co-trimoxazole in the first trimester and near term. Breastfeeding: vancomycin and clindamycin acceptable. Renal impairment: level-based vancomycin dosing, daptomycin every 48 hours when clearance is under 30 mL/min.

Decolonisation (nasal mupirocin twice daily for 5 days with chlorhexidine washing) is considered for recurrent infection, with limited evidence. Region: MRSA prevalence varies widely between East African hospitals; local data should decide empirical vancomycin, and where daptomycin and linezolid are unavailable, careful vancomycin dosing and source control matter more.

Referral and escalation. Involve infectious diseases early in all S. aureus bacteraemia, endocarditis, implant infection, CNS infection and persistent bacteraemia.